The digestive system
of ducktails and rostrids
To reduce interspecific competition, pygostylids and rostrids have developed two different types of digestive systems. While the firsts have evolved objectively the most efficient way to digest cellulose, the seconds were able to optimize nutrient assimilation thanks to behavioral adaptations.
Inside a ducktail
Ducktails, after chewing plants in the mouth with their keratinous tongue and false beak teeth, ingest the bolus that is further crumbled inside the crop, with the help of small rocks (gastroliths). The processed food moves then in the three-chambered gizzard, where cellulose is mostly digested and assimilated, thanks to a rich bacterial flora. Every chamber is separated by a valve, that helps to slow down the bolus.
The remaining bolus becomes chyme and then chyle, which is ultimately assimilated inside the two ceca.
Ducktails' poop is light brown and very homogenous, with variable water content depending on the species.
Some derived ducktails have completely changed their ancestral digestive system, like wardrums, which possess a very simplified gastrointestinal tract due to their hypercarnivorous diet: ceca are reduced, the gizzard is not chambered and the false teeth are not used to chew food, but to grasp it. Feces are extremely smelly and rich in undigested parts (bones, feathers, hairs, etc.), since wardrums to not produce pellets.
Inside a rostrid
Unlike ducktails, rostrids have lost their primordial false teeth to chew, in favor of a larger crop. After being ground in the crop, the bolus is slowly digested (but not assimilated) in the gizzard, thanks to bacteria and fungi with lignase and cellulase enzymes. The food is then partially assimilated inside the two enlarged ceca. The processed chyle is then defecated and eaten again. First poops are very heterogeneous, with green color, while redigested poops are darker and more uniform. In smaller species, like the dwarf mossbrusher, poops are sometimes assimilated twice, especially if rich in wood.
At least one known radiation of rostrids was able to optimize its diet by moving the main digestion area in the small intestine. Woodclimbers for example possess a long concamerate intestine, with small valves that slow down food to improve digestion.
The chyle that arrives in the ceca is on average 60% more assimilable than the one in other rostrid species and does not need to be re-ingested again.
A similar digestive system probably evolved independently several times in the rostrid clade, but we have little information about it.
